1. Field of the Disclosure
The disclosure relates to a filter. Particularly, the disclosure relates to a charge-domain filter (CDF).
2. Description of Related Art
A charge-domain filter (CDF) uses a filter technique of a switched-capacitor network (SCN). The SCN is a commonly used circuit in discrete time signal processing, which includes a plurality of switches and a plurality of capacitors. By turning on/off the switches, the CDF controls a charge storage status of the capacitors, so as to perform filter processing on an input signal. Compared to a capacitive-resistive analog filter circuit, a filtering effect of the SCN is mainly determined by size ratios of the capacitors therein. Even if an area of each capacitor is changed due to process variation, since the size ratios of the capacitors are ideally not changed along with the process variation, the filtering effect of the SCN is not changed. Therefore, the SCN is suitable to be fabricated in chips.
However, since the CDF may perform charge samplings, the process variation may lead to a gain variation and a direct current (DC) offset. If the DC offset is severe, a variable gain amplifier (VGA) of an analog to digital converter (ADC) in a receiver is hard to maintain a constant gain. On the other hand, a sampling pulse of the CDF relates to an input sampling rate, which means that a pulse width is inversely proportional to the input sampling rate. Therefore, based on a same finite impulse response (FIR) coefficient of the CDF, a narrow band CDF may cause a positive gain variation, and a wide band CDF may cause a gain loss.